The present invention relates to a method of fabricating a release substrate produced from semiconductor materials.
The term “release substrate” means a substrate comprising two layers which have been associated together via a reversible connection, the connection between the two layers being breakable, typically by mechanical action. Thus, the action of breaking the connection between these layers corresponds to “releasing” the substrate.
Methods of releasing a substrate are already known. As an example, the fabrication of release substrates by bonding two layers is known, where the bonding conditions are defined so that the bonding energy remains limited and for this reason, the bonding is reversible under the effect of mechanical action.
US patent application 2004/0222500 provides an example of such release substrates. The “energy” associated with a bonded interface, or more generally, with a releasing interface of a release substrate, is defined as corresponding to a cohesive force between the two layers defining that interface. That energy is generally characterized by mechanical resistance:                resistance to separating the two layers—typically under the effect of inserting a wedge attacking the interface, and/or        resistance to delamination (applying tension to one of the two layers defining the interface, and measuring the tensile stress corresponding to the onset of delamination).        
In many applications, treatments (for example anneal type heat treatments) have to be carried out on the release substrate prior to releasing it. It is known that heat treatment tends to increase the energy (i.e. the cohesive force) of a bonding interface between two layers. This is shown in prior art documents such as US patent applications 2001/0016399 or 2003/0008475 or U.S. Pat. No. 6,352,909 which disclose heat treatments either for providing a gettering layer in the wafer or for enhancing the quality of the bonding interface in a wafer made from bonded substrates.
One problem which arises with such release substrates is that any treatments (in particular heat treatments) which are to be carried out on the release substrate prior to release may compromise the reversible nature of the “release” bond of the substrate, rendering release problematical. Certain heat treatments can weaken the bond so that release occurs when not desired, while others can actually strengthen the bond making it more difficult to release the substrates when desired. These problems are not limited to release substrates in which the reversible connection is constituted by bonding. In general, applying an anneal type heat treatment to release substrates having a reversible connection that has been formed using any technique (bonding, and other techniques—for example, creating a porous region, etc) risks modifying the energy associated with the reversible connection of the substrate so that release does not occur when desired. This is a problem which is now solved by the present invention.